In recent years, it has been firmly established that transcriptional regulation plays a fundamental role in the developmental and physiological regulation of eucaryotic systems. In fact, it may be that genotypic variation governing phenotypic variation between species exists mostly at the transcriptional rather than at the structural level. We will study the mode and the frequency of transcriptional regulation evolution using as a model system the rich reservoir of eggshell phenotypic variants established in short time periods in the evolution of Hawaiian Drosophila. Specifically, we will gauge the extent of natural transcriptional variation by means of the characterization of the transcriptional regulation of various late minor chorion genes in several species exhibiting diverging chorion surface phenotypes. We will then identify a transcription variating gene whose developmental expression can be reproduced in a D. melanogaster background. This will lead to future experiments that will identify cis-acting regulatory elements as well as their mode of function and evolution. The fundamental approach is to use the natural transcription variants of the gene tb help select, by sequence analysis, candidate regulatory DNA elements. In vivo assays of mutagenized versions of these candidates will identify the elements responsible for the observed natural variation as well as their function in subsequent experiments. In addition, whole mount in situ hybridization of late minor chorion cDNAs to developing follicles will expand our knowledge on the number and distribution of their messengers, and will suggest functional significances for the natural transcription variants.